An outbreak of the Ebola virus in the Democratic Republic of Congo has now killed 31 people and could threaten major towns, the World Health Organization has warned.

An epidemic was officially declared on 17 August in the north-western Orientale Province.

WHO official Eugene Kabambi told Reuters that the situation was "very serious" and was "not under control".

Ebola is highly contagious and kills up to 90% of people infected.

There is no known treatment or vaccine for the disease, which is spread by close personal contact and causes massive internal bleeding.The death toll from this latest outbreak, centred on the towns of Isoro and Viadana, has more than doubled over the course of a week to 31.

Up to five health workers are thought to be among the dead.

"The epidemic is not under control. On the contrary the situation is very, very serious," Mr Kabambi warned, speaking in DR Congo's capital Kinshasa.

"If nothing is done now, the disease will reach other places, and even major towns will be threatened," he said.

Last month an outbreak of a more deadly Ebola strain in neighbouring Uganda killed 16 people, but health workers say the two outbreaks do not appear to be related.